1. Field of the Invention
The present invention relates to an amplifying circuit, and more particularly, to an amplifying circuit with high linearity and low power consumption.
2. Description of the Related Art
As well known by the person of ordinary skilled in the art, input/output stage circuits can be basically divided into three categories, that is, class A circuit, class B circuit, and class AB circuit. Where the performance of class AB circuit falls between the performance of the class A circuit and the class B circuit. In contrast to the class A circuit, the class AB circuit's power consumption is lower. Furthermore, in contrast to the class B circuit, the class AB circuit can provide an improved linear relationship between an amplified signal and an input signal.
For details about the related circuit, please refer to “A Pipelined 5-M Sample 9-bit Analog-to-Digital Converter” published in the December 1987 issue of the JSSC. Another paper titled “A High-performance Micropower Switched-Capacitor Filter” published in the December 1985 issue of the JSSC. Finally, the paper titled “A Programmable 1.5V CMOS Class-AB Operational Amplifier with Hybrid Nested Miller Compensation for 120 dB Gain and 6 MHz UGF” is found in the 1994 edition of the ISSCC.
In the paper titled “A Compact Power-efficient 3V CMOS Rail-to-Rail Input Output Operational Amplifier for VLSI Cell Libraries” published in the ISSSC in 1994, an operational amplifier circuit is disclosed. Please refer to FIG. 1. FIG. 1 is a circuit diagram of an operational amplifier 100 as disclosed in the said ISSSC's 1994 paper. As shown in FIG. 1, the operational amplifier 100 comprises a class A input stage circuit 110, a biasing circuit 120, and an output circuit 130, where the biasing circuit 120 and the output circuit 130 form a class AB output stage circuit.
In reference to FIG. 1, the static current Iq of the output circuit 130 should be appropriately designed such that the entire operational amplifier circuit 100 can be operated at a best operational point when it performs a signal amplifying operation. This means a better linear relationship can be achieved such that the signal will have a larger swing.
Please refer to FIG. 2. FIG. 2 is a diagram showing a characteristic curve of the output circuit 130 as shown in FIG. 1. In FIG. 2, Iq represents a static current when the input signal is a common-mode voltage (i.e., the differential voltage Vid is 0). IMAX and IMIN represent the maximum and minimum currents sustained by the output circuit 130 under the situation of input signal and output signal still keeping linearity. (That is, the transistors M25 and M26 operated in the saturation region).
As well known by the person of ordinary skilled in the art, in order to make the output signal achieve a maximum swing, the difference between IMAX and IMIN needs to be designed as large as possible. Please note that the swing can be equivalently regarded as an amplified degree without distortions. On the other hand, when there is no input signal (i.e., when the differential voltage Vid is 0), in order to reduce the power consumption, the static current Iq should be designed as small as possible.
However, the above-mentioned circuit cannot obtain the two advantages of amplified degree and the power consumption at the same time. Please note, the above-mentioned circuit uses the class A input stage circuit 110, which indicates that the current from the input stage circuit 110 to the biasing circuit 120 is determined by the current sources Ib1 and Ib2. Therefore, when the gate voltages of the transistor M19 and M20 are determined, the voltage difference VAB between the gate voltages of the transistors M25 and M26 and the static current Iq of the output stage circuit are also correspondingly determined at the same time. In other words, the operational point is determined. Finally, when the input signal is inputted into the operational amplifier 100, the operational point does not change (e.g., the above-mentioned voltage VAB and the static current Iq remain the same).
Therefore, to achieve reduced power consumption of the entire circuit 100, the static current Iq should be set to a smaller value. For example, this can be achieved through setting the gate voltages of the transistors M21 through M24). However, this action also influences the voltage difference VAB between the gate voltages of the transistors M25 and M26 such that the gate voltages of the transistors M25 and M26 are getting higher. In this way, the voltage differences between the gate and the source of the transistors M25 and M26 are also made smaller. As a result, the linearity of the output stage becomes worse and the maximum swing of the output signal is smaller.
Alternatively, if the maximum swing of the output signal is desired to be larger and a better linearity should be needed, the cross voltages of the transistors M21 through M24 should be larger. For example, adjusting the cross voltages of the transistors M21 through M24 to make the voltage difference between the gate and the source of the transistors M25 and M26 lower. However, in this way, when there is no input signal (i.e., the differential voltage Vid is 0), the static current Iq consumes more power.
From the above disclosure, it can be seen the power consumption and the signal swing cannot be optimized at the same time. It is apparent that a solution is needed.